1. Field of the Invention
The present invention relates to a manufacturing method of multilayer printed wiring boards, and more particularly, to a manufacturing method of multilayer printed wiring boards comprising polyimide laminated layers. Polyimide is known as an insulation material having a heat-resistant characteristic, and widely used for printed wiring boards. The manufacturing method of the polyimide multilayer printed wiring boards according to the invention is effective for preventing the printed wiring board from deterioration in a mechanical strength during manufacturing steps.
2. Description of the Related Art
High polymerized organic compounds (abbreviated briefly as polymers) such as polyimide, epoxy, polyester, and polycarbonate are widely used in electronic industry as an insulation material. Among them, polyimide is an extremely stable material against heat application. The polyimide does not decompose when heated up to a temperature of about 400.degree. C., therefore, the polyimide is known as a heat-resistant material and widely used for an insulation layer between multiple-laminated wiring layers of printed wiring board or semiconductor integrated circuit.
Hereinafter, the known manufacturing method of polyimide multilayer printed circuit boards is outlined.
Polyamic acid is known as one of precursors for producing polyimide by dehydration and ring closure. The polyamic acid having a molecular formula shown by the following formula (1) is known to be fluid and suitable for coating on a substrate. ##STR1##
The polyamic acid (1) is coated on a heat resistant insulation board of ceramic such as alumina. The ceramic board with the polyamic acid coating is dried and thereafter heated for curing, thereby the polyamic acid dehydrating and causing ring closure reaction forming a polyimide insulation layer shown by a molecular formula (2).
An electrically conductive layer is deposited on the polyimide insulation layer by sputtering or vacuum evaporation technologies. The conductive layer thus formed is subjected to the known photolithography processes, forming a patterned wiring layer on the polyimide insulation layer.
The ceramic board with the polyimide insulation layer and the patterned wiring layer thereon is once again subjected to the same steps of coating the polyamic acid. After drying and curing steps, a second polyimide insulation layer is formed. Via-holes are next formed by the conventional photolithography processes, which are necessary for connecting the underlying patterned wiring layer with an overlying wiring layer subsequently formed on the second polyimide insulation layer. If the polyamic acid is of an ultra-violet ray sensitive type, the processes for forming the via-holes are simplified, whereby necessary regions of the second polyamic layer are selectively exposed to ultra-violet rays and, after a developing process, the polyamic layer is heated for curing, forming the second polyimide layer with the via-holes.
An electrically conductive layer is deposited on the second polyimide layer by sputtering or vacuum evaporation technologies and thereafter subjected to the photolithography processes, forming a second patterned wiring layer on the second polyimide layer in the same way.
These steps are repeated necessary times for obtaining a required number of polyimide layers and wiring layers of the printed wiring board.
The polyimide multilayer printed wiring board of the prior art as described above has a weak point that an embrittlement characteristic of the polyimide layers deteriorates. Mechanical strength of polyimide layers of the multilayer printed wiring board becomes weaker than that of a polyimide single-layer printed wiring board. In order to enhance reliability of the polyimide multilayer printed wiring boards, improvement in the embrittlement characteristic is required.